Pee Pee Help, Please

I am so tired of washing sheets. Little Man, who has been potty trained for quite some time now... maybe 2 years now, still isn't able to make it through the night and occasionally will pee during naps (for which he does not wear a diaper or pullup). So... What the heck do I do? Keep changing sheets until he outgrows this? We tried doing no diapers at night but he doesn't wake up and is truly baffled when he wakes up and his bed is wet. So, clearly he is not coherant enough to get out of bed and go potty by himself at night. I have changed the sheets on his bed 5 times in the last week... and I am usually lucky if I change them once every 5 weeks. ;) HELP!